Re: BGP prefix filter list

[Blake Hudson <blake () ispn net>]

>     I would argue that one can generally safely add information to his
>     or her router's RIB (such as adding a local preference, weight, or
>     advertising with prepends to direct traffic toward a better
>     performing, less utilized, or lower cost peer), but that removing
>     information from a router's RIB always comes at some cost (and
>     some may find this cost perfectly acceptable).
>
>
> One needs to remember that removing information from RIB is how BGP 
> works. If you have the common setup of two BGP edge routers, each with 
> a directly connected transit provider link, the routers will only tell 
> the other one about the routes it actually uses. Neither router has a 
> complete view.
I manage a network like you describe: Two BGP edge routers, both routers 
accept a full eBGP feed from transit, both share routing information via 
iBGP. Both edge routers in my network have a complete view. If one 
transit provider is down or there is an upstream peering change, both 
still have a complete view. The only time they wouldn't have a complete 
view is during convergence or when there is a simultaneous outage of 
both transit providers at different physical facilities.

I could certainly use a default route (configured statically or received 
via BGP) instead, but that reduces my network's ability to make informed 
decisions. When one of my upstream transit providers is performing 
maintenance and loses a peer, I want that to be reflected in my routing 
so that traffic can be directed via the shortest path. When my transit 
provider's edge router loses upstream connectivity, but maintains 
connectivity to my equipment, I want that reflected in my routing so 
that traffic doesn't go towards the path that leads to the bit bucket. I 
can't detect those conditions and route around them if my router only 
has a default route.